The present invention generally relates to magnetic storage devices, and more particularly to an improvement of control of magnetic heads in the so-called hard disk devices.
Hard disk devices are used extensively as a large capacity, high speed auxiliary magnetic storage device of computers. A typical hard disk device includes a rigid magnetic disk revolving at a high speed and a magnetic head carried on a swing arm for scanning the recording surface of the magnetic disk at a high speed. In response to the swinging motion of the swing arm, the magnetic head scans the recording surface of the magnetic disk generally in the radial direction thereof. Generally, the magnetic disk is revolved at a high speed in the order of several thousand r.p.m., and the magnetic head achieves the recording and reproduction of information signals on and from the magnetic disk while being separated therefrom by a thin air foil.
A typical hard disk device includes a number of magnetic disks mounted on a common hub for simultaneous revolution, and each magnetic disk includes a cooperating magnetic head that is held on a cooperating swing arm. There are a plurality of swing arms and plurality of magnetic heads in correspondence to the plurality of magnetic disks, and the foregoing plurality of swing arms are formed as a unitary body and held rotatably about a common swing axis. As a result, the foregoing plurality of magnetic heads, held on the respective swing arms, scan the surface of the magnetic disks simultaneously.
In such a conventional hard disk device, in order to achieve a proper tracking of the magnetic head for each of the magnetic disks, a magnetic servo pattern is recorded on one of the plurality of magnetic heads, and the magnetic head that cooperates with the magnetic disk on which the magnetic servo pattern is recorded, is controlled to trace the track pattern defined in the form of the magnetic servo pattern. As the other magnetic heads, cooperating with the other magnetic disks, are held unitarily to the magnetic head that traces the magnetic servo pattern, the tracking of magnetic heads is also achieved in the other magnetic disks.
FIG. 1 shows the internal construction of such a conventional hard disk device in plan view, wherein the left side of the drawing shows the state wherein the upper cover of the device is removed, while the right side shows a magnetic disk 11 forming a part of a multiple-stack magnetic disk assembly 10 and an arm assembly 12 that cooperates thereto.
Referring to FIG. 1, each magnetic disk 11 is mounted on a hub 11a that is driven by a motor not illustrated, and the arm assembly 12 includes a swing arm 12b held rotatably about an axis 12a and a magnetic head 12c provided on a free end of the arm 12b. Further, the arm 12b carries a coil 12d forming a part of a voice coil motor 13 at an opposite free end thereof. There, it will be noted that the coil 12d is wound in a plane substantially parallel to the scanning surface of the arm 12b, and magnets 13a and 13b forming a part of the voice coil motor 13 are disposed respectively at the upper side and the lower side of the coil 12d. Thereby, the energization of the coil 12d causes a swinging motion of the arm 12 about the axis 12a, and the energization of the coil 12d is controlled such that the magnetic head 12c on the arm 12b traces a track 11b defined on the magnetic disk 11.
FIG. 2 shows the internal structure of the hard disk device of FIG. 1.
Referring to FIG. 2, the multiple-stack magnetic disk assembly 10 includes a plurality of magnetic disks 11.sub.1, 11.sub.2, . . . held commonly on the rotary hub 11a, and in correspondence thereto, the arm assembly 12 is formed as an assembly of a number of arms 12b. Each arm 12b is held on a common rotary member 12e that is held rotatable about the axis 12a, and the arms 12b cause a simultaneous rotation in response to the rotational motion of the rotary member 12e, which of course is formed as a result of energization of the member 12e. Further, the entire mechanism of the hard disk device is accommodated in a hermetically sealed container 1.
In the conventional hard disk device having such a construction, one of the magnetic disk forming multiple-stack magnetic disk assembly 10 such as a magnetic disk 11.sub.1 is recorded with a magnetic servo pattern in correspondence to the track that is defined thereon. Thereby, the side of the disk 11.sub.1 that carries the servo pattern acts as a servo surface. Thus, by controlling the magnetic head that cooperates with the servo surface of the magnetic disk 11.sub.1 to follow the servo control pattern thereon, one can achieve the tracking also for other magnetic heads cooperating with other magnetic disks or other recording surfaces.
In the recent hard disk devices wherein the recording density is increased significantly, the interval between the tracks is reduced and the control of the magnetic head based upon the foregoing servo pattern on the servo surface alone tends to be insufficient. More particularly, the environment, particularly the temperature, in which the device is used, tends to influence the tracking control and there is a substantial risk that the magnetic head may offset from the proper track even when controlled properly based upon the servo pattern on the servo surface.
In order to remedy the foregoing problem of off-track and to achieve a more reliable positional control of the magnetic heads, there is a technique to record a servo pattern also on the recording surface of individual magnetic disks, such that the servo control is achieved based upon the disk servo pattern in addition to the servo pattern on the servo surface. There, the positional deviation of the magnetic head on the individual magnetic disk is corrected or calibrated with respect to the servo pattern on the servo surface based upon the disk servo pattern, and a correct tracking is guaranteed on each of the magnetic disks.
FIG. 3 shows an example of the disk servo pattern that may be provided at the peripheral region of the magnetic disk. Alternatively, such a disk servo pattern may be provided on a particular sector defined on the magnetic disk.
Referring to FIG. 3, the disk servo pattern includes a number of mutually isolated pattern elements SP disposed alternately at both sides of a line I that coincides to a track on the magnetic disk. There, each pattern element SP includes an alternate repetition of mutually opposing magnetic polarizations repeated in the rotational direction of the magnetic disk, and the offset of the magnetic disk with respect to the proper track is detected based upon the electric signal reproduced by the magnetic head upon scanning the pattern elements SP.
FIG. 4(A) shows the waveform of the electric signal reproduced in response to the disk servo pattern by a magnetic head that is properly centered on the track I, wherein it will be noted that the electric signal reproduced in response to the pattern elements SP located above the line I in FIG. 3 and the electric signal reproduced in response to the pattern elements SP located below the line I in the illustration of FIG. 3 have the same amplitude. On the other hand, when the magnetic head is offset from the proper center I of the track for example in the upward direction in FIG. 3, the electric signal reproduced in response to the pattern element SP located above the line I has an amplitude much larger than the electric signal reproduced in response to the pattern element SP located below the line I as indicated in FIG. 4(B).
FIGS. 5(A) and 5(B) show the level of the reproduced electric signal corresponding to the magnetization of the disk servo pattern SP described above, wherein it will be noted that FIG. 5(A) shows the on-track state corresponding to FIG. 4(A) while FIG. 5(B) shows the off-track state corresponding to FIG. 4(B). As the meaning of FIGS. 5(A) and 5(B) is obvious from FIGS. 4(A) and 4(B), further description will be omitted except that the magnetic head is represented as DH in FIGS. 5(A) and 5(B).
Thus, by detecting the electric signals reproduced by the magnetic head in response to the pattern elements SP, one can detect and correct the offset of the magnetic head with respect to the track. For example, the offset thus detected is stored in a memory for subsequent compensation for the head position. There, the position of the magnetic head that is specified based upon the servo pattern on the servo surface is corrected by the offset stored in the memory. Such a correction may be achieved periodically for compensating for the temperature-induced variation of the magnetic head. The recording of the disk servo pattern is achieved in a stabilized state of the apparatus, for example after running continuously for several hours. Thereby, the disk servo pattern SP is written at both sides of the track center I symmetrically.
In the hard disk device of the foregoing type, the detection of positional deviation of the magnetic head is achieved properly as long as the magnetic head has a symmetrical recording and reproducing characteristics as shown in FIGS. 5(A) and 5(B). On the other hand, when the magnetic head is mounted on the swing arm with a slight distortion or when there is an error in the fabrication of the magnetic head, there can occur a case shown in FIG. 6(A) wherein the level of magnetization in each servo pattern element SP is asymmetric with respect to the radial direction of the magnetic disk. When this is the case, the reproduced electric signal corresponding to the pattern SP at the left of the line I of FIG. 6(A) takes a larger amplitude as compared to the reproduced electric signal that corresponds to the pattern SP at the right of the line I, even when the magnetic head is positioned exactly on the track center I. Thereby, the reproduced electric signal has a waveform as shown in FIG. 6(B) that is similar to the waveform of FIG. 4(B) that represents the off-track of the head.
Thus, the conventional hard disk device has a problem of erroneously compensating for the deviation of the magnetic head based upon the false disk servo pattern PL.